Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven”) or through the use of a gearbox. Direct driven wind turbines also may be “shaftless”; in these shaftless wind turbines, the rotor directly drives the generator.
Direct drive wind turbines such as known from e.g. WO 2005/103489, do not suffer from known problems related to the gearbox. However, since there is no speed increase, the generator shaft rotates very slowly. As a consequence, a large and relatively expensive generator is generally needed to be able to generate electricity in an effective way. Additionally, when bending loads and movements (and corresponding deformations) are transmitted through the rotor shaft to the generator, it may not be possible to control the air gap between generator rotor and generator stator. High bending loads can even cause structural damage to parts of the generator, e.g. its bearings. Replacement or repair of such generator parts may be very expensive due to the size and related cost of the generator and its components.
WO 01/59296 discloses a direct drive wind turbine comprising a hub with a plurality of blades, the hub being rotatably mounted relative to an axle part. The hub of the turbine is connected to the generator rotor by means of a plurality of connecting members, which are torsion stiff but yielding to bending moment.
With this kind of configuration the loads due to the weight of hub and blades are transmitted more directly via the frame to the tower, whereas the rotor transmits mainly torque to the generator, thus substantially reducing undesired deformations in the generator. This may represent an improvement with respect to other prior art wind turbines, but the transmission of bending loads from the rotor to the generator cannot be avoided entirely.
There thus still exists a need for a direct drive wind turbine, wherein the transfer of bending loads and movements from the rotor hub to the generator can substantially be reduced. There also still exists a need for a direct drive wind turbine in which an air gap can be more effectively controlled, particularly under extreme load cases such as those resulting from e.g. wind gusts.
It is an objective of the present invention to at least partially fulfil these needs. Further advantages of the present invention will become apparent from its description.